Method for preparing a resin overcoated electrophotographic plate

ABSTRACT

A method of preparing an electrophotographic plate which comprises coating a resin upon the surface of an amorphous selenium layer while permitting the selenium to devitrify, curing said resin, heating the selenium layer to about its softening temperature, and quenching said layer is disclosed. Electrophotographic processes employing said plate are also disclosed.

United States Patent METHOD FOR PREPARING A RESIN OVERCOATED ELECTROPHOTOGRAPIIIC PLATE 7 Claims, 1 Drawing Fig.

U.S. Cl. 96/1,

96/1.5,l17/17.5,117/218,l17/34 Int. Cl G03g 13/22 Field of Search 96/1, 1.5;

[56] References Cited UNITED STATES PATENTS 2,886,434 5/1959 Owens 96/1.5 3,248,261 4/1966 Narken et a1. 1 17/201 3,442,822 5/1969 Kim 117/201 2,662,832 12/1953 Middleton et al. 23/209 2,663,636 12/1953 Middleton 23/209 3,140,174 7/1964 Clark 96/1.8

Primary ExaminerN0rman G. Torchin Assistant Examiner-Judson R. Hightower Altorneys-Stanley 2. Cole and James J. Ralabate ABSTRACT: A method of preparing an electrophotographic plate which comprises coating a resin upon the surface of an amorphous selenium layer while permitting the selenium to devitrify, curing said resin, heating the selenium layer to about its softening temperature, and quenching said layer is disclosed. Electrophotographic processes employing said plate are also disclosed.

PATENIEU Nnv2 |97| FORM SELENIUM CONTAINING LAYER OVERCOAT WITH RESIN CURE RESIN HEAT TO MELT SELENIUM CONTAINING LAYER QUENCH A TTORNEVS METHOD FOR PREPARING A RESllN OVERCOATED ELECTROPHOTOGRAPHIC PLATE This invention relates to electrophotography and, more specifically, to improved electrophotographic plates and methods of preparing and using them.

it is known that images may be formed and developed on the surface of certain photoconductive materials by electrostatic means. The basic electrophotographic process, as taught by Carlson in U.S. Pat. No. 2,297,691, involves uniformly charging a photoconductive insulating layer and then exposing the layer to a light-and-shadow image which dissipates the charge on the portions of the layer which are exposed to light. The electrostatic latent image formed on the layer corresponds to the configuration of the lightand-shadow image. Alternatively, a latent electrostatic image may be formed on the plate by charging the plate in image configuration. This image is rendered visible by depositing on the imaged layer a finely divided developing material comprising a colorant called a toner and a toner carrier. The powdered developing material will be attracted to those portions of the layer which retain a charge, thereby forming a powder image corresponding to the latent electrostatic image. This powder image may then be transferred to paper or other receiving surfaces. The paper then will bear the powder image which may subsequently bemade permanent by heating or other suitable fixing means. The above general process is also described in U.S. Pat. Nos. 2,357,809; 2,891,011 and 3,079,342.

That various photoconductive insulating materials may be used in making electrophotographic plates is known. Suitable photoconductive insulating materials such as anthracene, sulfur, selenium, or mixtures thereof, have been disclosed by Carlson in U.S. Pat. No. 2,297,691. These materials generally have sensitivity in the blue or near ultra-violet range, andall but selenium, have a further limitation of being only slightly light sensitive. Because of its greater sensitivity, selenium has been the most commercially accepted material for use in electrophotographic plates. Vitreous selenium, however, while desirable in most aspects, suffers from serious limitations in that the surface of the selenium is subject to degradation from heat and/or certain organic solvents. The vitreous selenium surface may be vitrified producing the unsuitable crystalline form. While the vitreous selenium surface is fairly resistant to abrasion, after extendeduse imagedegradation occurs clue to surface wear and surface scratching.

Recently several different types of materials have been proposed for use in photoconductive insulating layers used in electrophotographic plates. For example, the use of inorganic photoconductive pigments dispersed in suitable binder materials to form photoconductive insulating layers is known. Also, certain organic photoconductors are known which may be formed into homogeneous films or may be dispersed in a binder for use in photoconductive plates have been developed. These materials, however, are generally inferior to selenium from the point of view of sensitivity and reusability. in general, the organic photoconductors have much lower hotosensitivity than selenium. Many of the materials have high fatigue characteristics, i.e.,and inability to hold an electrostatic charge after one cycle of charging, exposing and developing. This makes these materials unsuitable for systems in which the plate is reused to form successive images. A number of these photoconductive insulating layers have low heat distortion properties which make them undesirable in automatic electrophotographic apparatus which often includes powerful lamps, thermal heating devices which tend to heat the electrophotographic plate. Thus, selenium remains the preferred material for use in reusable plate electrophotography.

Attempts have been made to protect surface of selenium electrophotographic plates by overcoating them with a thin layer of an organic resin. Known overcoating-materials include the waxes and insulating resins such as are described by Dessauer et al. in U.S. Pat. No. 2,901,348; polyvinyl acetyl resins as described by Deubner in U.S. Pat. No. 2,860,048 and the highly polymerized transparent electrically insulating resins described by Kinsella in U.S. Pat. No. 3,146,145. Many of these overcoatings have been found to be capable of improving the surface characteristics of a selenium electrophotographic plate and in decreasing abrasive wear and solvent recrystallization. However, some of these materials have been found to having insufficient adhesionto the selenium surface and to chip or flake after prolonged use. Others, especially the waxes, are not in themselves sufficiently abrasion resistant and wear off rapidly. With many resins, for optimum adhesion and final overcoating toughness, it is preferred that they be applied either from a melt or from an organic solvent; which solvent or heat tends to crystallize the amorphous selenium layer to the unsuitable crystalline form. Further, many resins attain optimum physical characteristics only after prolonged heat curing at relatively elevated temperatures. These elevated temperatures have also been found to devitrify the amorphous selenium photoconductive layer. Thus, there-is a continuing need to provide improved protective layers for selenium electrophotographic plates and improved methods of forming protective layers without degrading the selenium layer.

It is therefore, an object of this invention to provide a method of treating the surface of an electrophotographic plate which overcomes the above noted disadvantages.

Another object of this invention is to provide an electrophotographic plate with a protective overcoating having high desirable physical properties.

Another object of this invention is to provide an electrophotographic plate having high resistance to abrasion and scratching.

Still a further object of this invention is to provide a method of preparing a electrophotographic plate having a wide range of useful physical properties.

Still a further object of this invention is to provide a method of overcoating a selenium plate with a heat-settable resin while maintaining the selenium in the desirable amorphous condition.

The foregoing objects and others are accomplished in accordance with thisinvention, generally speaking, by providing a method for overcoating an electrophotographic plate which comprises the steps of coating a resin onto the surface of an amorphous selenium layer, while permitting the selenium to devitrify, curing said resin, heating said plate to the softening temperature of the selenium and quenching said plate whereby said selenium layer is returned to the amorphous state. Surprisingly, the adhesion between the resin overlayer and the selenium layer remains exceptionally high through the heating and quenching steps.

If desirable, the resin may be dissolved in a solvent which devitrifies selenium. In this embodiment, the solution is coated onto the surface of the selenium layer without any precautions to prevent devitrification of the selenium. The plate is 'then heated to drive off the solvent, drying the resin overlayer. Heating is continued to the softening temperature of the selenium whereupon the plate is quenched to a temperature below the softening point of the selenium. The selenium layer is thus-returned to the amorphous state. Thus, resins which are soluble only in solvents which devitrify selenium may be used in this process where as in previous processes the adverse effects prevented their use.

An alternative embodiment, a thermo-setting resin may be applied to the surface of the selenium plate. Here the plate is heated to cure the resin and then heating is continued until the softening temperature of the selenium is reached. Again, the selenium plate is quenchedto return the selenium layer to the amorphous state. Before quenching, the plate is heated to about the softening temperature of the selenium containing layer. The melting point of the crystalline form is about 220 C. However, the plate may be heated to any suitable temperature, which may vary with additives, etc. Typically, the temperatures will range from about 200 C. to about 300C. The quench bath may comprise any suitable material at any suitable temperature. Typically, water at about room temperature has been found to produce excellent results.

The overcoating material may be applied to the surface of the selenium plate by any suitable method without regard to adverse effects on crystallinity of the selenium layer. Typical application methods include coating from a solution of the resin in a solvent such as by dip coating, spraying or Mayer bar draw down; polymerization of a monomer in situ on the selenium surface; coating from a melt; formation of a layer of powdered resin followed by sintering; electrostatic spraying; or any desirable combination thereof. After the layer is formed, it may be further polymerized or cross-linked by chemical or thermal means, where desirable.

The overcoated selenium layer may be self-supporting or may be coated on a conductive substrate. Said substrate may comprise any suitable conductive material having the capability of acting as a ground plane for the electrophotographic plate. Typical conductive materials include metals such as aluminum, brass, stainless steel, copper, nickel, and zinc; conductively coated glass such as tin oxide, indium oxide or aluminum coated glass; similar coatings on resin substrates; or paper rendered conductive by the inclusion of a suitable chemical therein or conditioning in a humid atmosphere to ensure the presence therein of a sufficient water content to render the material conductive.

The selenium layer referred to above may contain in addition to selenium other suitable materials where desired. This layer may be a selenium alloy or a mixture of other materials with selenium. Typical selenium containing mixtures or alloys include cadmium selenide, cadmium sulfo-selenide, mixtures of sulfur and selenium such as described by Carlson in U.S. Pat. No. 2,297,691; mixtures of arsenic and selenium such as are described by Mayer et al. in U.S. Pat. No. 2,822,300; mixtures of selenium and tellurium as described by Paris in U.S. Pat. No. 2,803,541; arsenic selenide; tellurium selenide; and mixtures thereof. If desired, the selenium layer may include various sensitizers or dopants; such as the halogens as described in copending application Ser. No. 516,529 filed Dec. 27, 1965. While it is known in the art to include additives in a selenium photoconductive layer to inhibit crystallization additives are in general unnecessary in a plate prepared by the process of the present invention since the above described quenching step returns the selenium layer to a uniform amorphous state after the overcoating layer is formed and cured. Such additives, however, may be included where desired to inhibit later crystallization. The selenium containing layer may have any suitable thickness. Where the photoconductive layer is to be self-supporting, the selenium layer will be relatively thick for mechanical strength. Where the selenium containing layer is coated onto a conductive substrate, it is preferred that the layer have a thickness of from about 5 to 50 microns for optimum sensitivity and thermal and mechanical adhesion to the substrate.

The overcoating layer may comprise any suitable insulating resin material which may be coated from any suitable solvent and which may be cured or set at any temperature up to the melting temperature of the selenium containing layer. Typical insulating resins include polyolefins such as polyethylene, polypropylene; vinyl and vinylidene polymers such as polyvinylchloride, polyvinyl carbazole; polystyrene; fluorocarbon resins such as polytetrafluoroethylene, polyvinylfluoride; polyamides such as polycaprolactam; polyesters such as polyethyleneterphthalate; polyurethanes; polypeptides such as casein; polysulfides; polyphenylene oxide; polysulfones; polycarbonates; cellulosic polymers such as viscose, cellulose acetate; phenolic resins such as phenolformaldehyde; amino resins such as melamine-formaldehyde; alkyd resins; alkyl resins; epoxy resin; and mixtures and copolymers thereof.

The invention may be further understood upon reference to the drawing which shows a flow sheet for the process of this invention.

As shown in the Figure, first a photoconductive insulating layer containing selenium is formed. The surface ofthe plate is then overcoated with a'thermosetting resin or with a resin dissolved in a solvent which recrystallizes selenium. Next the overcoating is cured, either by heating to a curing temperature or by evaporating off the solvent. No precautions need be taken to prevent crystallization of the selenium. Then the plate is heated to the softening temperature of the selenium containing layer and the plate is quenched to return the selenium to the amorphous state.

For optimum plate sensitivity with lowest residual charge after exposure, it is preferred that the overcoating layer have a thickness of up to about 1 micron. If desired, the overcoating layer may include other materials such as electrical or dye sensitizers or photoconductive materials such as phthalocyanine.

The following examples will further define the present invention with relation to the improved process for preparing an electrophotographic plate. Parts and percentages are by weight unless otherwise indicated. The examples below should be considered to represent preferred embodiments of the present invention.

EXAMPLE l Two plates are prepared by vacuum evaporating selenium onto two 4 inch square pieces of 5 mil aluminum sheet to a selenium thickness of about 20 microns as described by Bixby in U.S. Pat. No. 2,970,906. Onto each plate is flow coated a film of an epoxy phenolic resin solution. This solution is prepared by dissolving about 35 parts Epon-l007, an epoxy resin available from the Shell Chemical Co., about 20 parts Methylon-75201, a phenol-formaldehyde resin available from the General Electric Co., and about 4 parts Uformite F-240, a urea-formaldehyde resin available from the Rohm & Haas Co., in a mixture of about 30 parts methyl isobutyl ketone and about 10 parts methyl ethyl ketone. The plates are then heated to about 200 C. for about 30 minutes to remove residual solvent and cure the resin. The overcoating thickness is about 1 micron. One of the two plates is then heated to about 260 C. on a hot plate and quenched by dropping into water at about 30 C. After about 1 minute, the plate is removed from the water and dried. Each plate is found to have a very tough, abrasion resistant surface. Each of these plates is then electrostatically charged in the dark by means of a corona discharge means held at a potential of about 6,000 volts as described by Carlson in U.S. Pat. No. 2,588,699. The quenched plate is found to hold a potential of about 650 volts while the unquenched plate holds a potential of about volts. Each plate is then exposed to a black-and-white image by means of a conventional transparency in a Simmons Omega Model D Enlarger. Total exposure is about 15 foot candle seconds. The resultant latent electrostatic images are developed by cascading a mixture of carrier beads and toner particles across the plate as described by Walkup in U.S. Pat. No. 2,618,551. A powder image is observed on the quenched plate conforming to the original. Very little toner is observed to remain on the unquenched plate. The powder on each plate is transferred to paper receiving sheets by the method described by Schaffert in U.S. Pat. No. 2,576,047. The image produced by the quenched plate is found to be of excellent quality conforming to the original. The image produced by the unquenched plate is found to be of very low quality. The selenium layer on each of the two plates is analyzed by X-ray powder diffraction. The selenium on the quenched plate is found to be substantially entirely amorphous in character. The selenium in the unquenched plate is found to be substantially entirely crystalline. As shown by the above example, the unquenched plate is largely incapable of holding an electrostatic charge and thus produces very poor images.

EXAMPLE ll Two plates are prepared as follows:

a. a 50 micron layer of amorphous selenium is coated onto an aluminum substrate by vacuum evaporation. A 1 micron dry thickness layer of the epoxy-phenolic resin described in example I is coated onto the selenium surface. The resin is cured by maintaining the plate at about 100 C. for about 1 hour.

b. a 50 micron layer of amorphous selenium is coated onto an aluminum substrate by vacuum evaporation. A layer of the epoxy-phenolic resin is then formed on the selenium surface to a dry thickness of about 1 micron. The resin is cured by maintaining the plate at about 200 C. for about 30 minutes. This plate is then heated to about 260 C. and quenched in cool water. The plate is then removed from the water and dried.

Each of these plates is cycled through steps of electrostatic charging, development by cascade, transfer of powder images to a receiving sheet, cleaning by means of a fiber brush and recharging. After about 50,000 repetitions of the charge expose and develop cycle the surfaces of the plates are examined under a microscope. The EPA surface which had been cured at 100 C. is found to be noticeably worn with significant scratches across the surface. The other plate which had been cured at 200 C. then heated to the softening temperature of the selenium and quenched is found to have much less wear and fewer scratches. The more completely cured resin surface is thus found to be noticeably more durable.

EXAMPLE Ill Selenium is coated onto an aluminum sheet to a thickness of about 60 microns. This layer has the glossy black appearance characteristic of vacuum deposited amorphous selenium.

About 1 part Pyre ML RK-692, an aromatic polyimide resin available from E. l. du Pont de Nemours & Company is dissolved in about parts dimethylformamide. This solution is spray coated onto the selenium surface. The plate is maintained at about 200 C. for about 1 hour to cure the resin. Dry film thickness is about 0.5 micron. The selenium surface as observed through the overcoating has the grey-black color of crystalline selenium, indicating thermal conversion to the unsuitable crystalline form. The plate is then heated to about 260 C. and quenched in water at room temperature. The selenium now has regained the appearance of amorphous selenium. The plate is then charged, exposed and developed as in example I. An image of good quality corresponding to the original results.

EXAMPLE IV A 50 micron amorphous selenium layer is formed on an aluminum substrate. About 1 part Uformite F-240, a ureaformaldehyde resin available from Rohm & Haas Co. is dissolved in a mixture of about 5 parts xylol and about 5 part butanol. This solution is dip coated onto the selenium surface. The plate is maintained at about 160 C. for about 2 hours to cure the resin. Dry coating thickness is about 0.8 micron. The selenium appears to have been converted to the crystalline form. The plate is then heated to the softening temperature of selenium and is quenched in water at room temperature, The selenium appears to have returned to the amorphous state.

EXAMPLE V A 40 micron layer of amorphous selenium is formed on an aluminum substrate. About 1 part Merlon, a polycarbonate resin available from Mobay Chemical Co. is dissolved in about l0 parts diethylene triamine. This solution is coated onto the selenium surface. The plate is maintained at about 80 C. for about 1 hour to remove residual solvent. The dry coating has a thickness of about 1 micron. The selenium surface appears to have been converted by the solvent to the crystalline form. The plate is divided into two portions, one of which is heated to about 260 C. and is quenched in water at room tempera ture. The selenium on this portion appears to have returned to the amorphous state. Each of the two portions is charged, exposed and developed as in example I. An excellent image is observed on the quenched plate. A very poor, blotchy image is observed on the nonquenched plate.

EXAMPLE VI A plate is prepared comprising a photoconductive layer on an aluminum substrate by the method described by Mengali in US. Pat. No. 2,745,327. The photoconductive layer comprises about percent amorphous selenium and about 5 percent tellurium.

A coating solution is prepared by dissolving about 70 parts Epon 1007, an epoxy resin available from the Shell Chemical Co. in about l20 parts Ethyl Cellosolve, an ethylene glycol monethyl ether available from Union Carbide Corp. To this solution is added about 10 parts Uformite F-240, a urea-formaldehyde resin available from the Shell Chemical Co. and about 40 parts Methylon 7520l, a phenol-formaldehyde resin available from the General Electric Co. This solution is spray coated onto the surface of the photoconductive layer. The plate is heated to about 200 C. for about 1.5 hours to cure the overcoating. The selenium in the photoconductive layer appears to have substantially crystallized. The plate is divided into two portions, one of which is heated to about 270 C. and is quenched in water at room temperature. Each portion is then charged, exposed and developed as in example i. An excellent image is found on the quenched plate, while a poor, blotchy image is found on the unquenched plate.

Although specific materials and conditions'were set forth in the above examples relating to methods of preparing electrophotographic plates, these were merely illustrative of the present invention. Various other compositions, such as the typical materials listed above and various conditions, where suitable, may be substituted for those given in the examples with similar results. The photoconductive insulating layer and/or coating layer of this invention may have othermaterials mixed therewith to enhance, sensitize, synergize or otherwise modify their properties. For example, the overcoating layer may have various photoconductive or sensitizing materials added thereto. Also, the selenium containing layer may include various additives, such as small amounts of a halogen, arsenic or tellurium.

Many other modifications of the present invention will occur to those skilled in the art upon a reading of this disclosure. These are intended to be encompassed within the spirit of this invention.

What is claimed is:

l. A process for preparing an electrophotographic plate which comprises the steps of:

a. providing a photoconductive insulating layer comprising selenium;

b. forming on a surface of said photoconductive insulating layer a second layer comprising a substantially insulating organic resin;

0. curing said resin to form a hard, durable surface;

d. heating said photoconductive insulative layer so that the selenium contained therein is softened; and

e. quenching said photoconductive insulating layer so that said selenium is present substantially entirely in the amorphous state.

2. The method of claim 1 wherein said resin is cured by heating said plate to a temperature at which said selenium containing layer at least partially crystallizes.

3. The method of claim 1 wherein said resin is coated onto said photoconductive insulating layer from a solvent which at least partially recrystallizes said selenium.

4. The method of claim 1 wherein said overcoating has a thickness of up to about 1 micron.

5. The method of claim 1 wherein said photoconductive insulating layer is formed on a conductive substrate.

6. An imaging process which comprises the steps of:

1. preparing an electrophotographic plate by the process of claim I;

2. forming an electrostatic latent image on the resin surface of said plate; and

3. developing said electrostatic latent image with electroscopic marking material whereby a visible image conforming to said electrostatic latentim age results.

7. The process of claim 6 wherein said electrostatic latent image is formed by substantially uniformly electrostatically charging said resin surface and exposing said charged surface to a pattern of activating electromagnetic radiation. 

2. The method of claim 1 wherein said resin is cured by heating said plate to a temperature at which said selenium containing layer at least partially crystallizes.
 2. forming an electrostatic latent image on the resin surface of said plate; and
 3. developing said electrostatic latent image with electroscopic marking material whereby a visible image conforming to said electrostatic latent image results.
 3. The method of claim 1 wherein said resin is coated onto said photoconductive insulating layer from a solvent which at least partially recrystallizes said selenium.
 4. The method of claim 1 wherein said overcoating has a thickness of up to about 1 micron.
 5. The method of claim 1 wherein said photoconductive insulating layer is formed on a conductive substrate.
 6. An imaging process which comprises the steps of:
 7. The process of claim 6 wherein said electrostatic latent image is formed by substantially uniformly electrostatically charging said resin surface and exposing said charged surface to a pattern of activating electromagnetic radiation. 